In a power transmission or distribution network, switching apparatuses are incorporated into the network to provide automatic protection in response to abnormal load conditions or to permit opening or closing (switching) of sections of the network. The switching apparatus may therefore be called upon to perform a number of different operations such as interruption of terminal faults or short line faults, interruption of small inductive currents, interruption of capacitive currents, out-of- phase switching or no-load switching, all of which operations are well known to a person skilled in the art.
In switching apparatuses the actual opening or closing operation is carried out by two contacts where normally one is stationary and the other is mobile. The mobile contact is operated by an operating device which comprises an actuator and a mechanism, where said mechanism operatively connects the actuator to the mobile contact.
Actuators of known operating devices for medium and high voltage switches and circuit breakers are of the spring operated, the hydraulic or the electromagnetic type. In the following, operating devices will be described operating a circuit breaker but similar known operating devices may also operate switches.
The spring operated actuator, or spring drive unit as it also is called, generally uses two compression springs for operating the circuit breaker; an opening spring for opening the circuit breaker and a closing spring for closing the circuit breaker and re-loading the opening spring. The closing spring is recharged by an electrical motor which is situated in the operating device. A mechanism converts the motion of the springs into a translation movement of the mobile contact. In its closed position in a network the mobile contact and the stationary contact of the circuit breaker are in contact with each other and the opening spring and the closing spring of the operating device are charged. Upon an opening command the opening spring opens the circuit breaker, separating the contacts. Upon a closing command the closing spring closes the circuit breaker and, at the same time, charges the opening spring. The opening spring is now ready to perform a second opening operation if necessary. When the closing spring has closed the circuit breaker, the electrical motor in the operating device recharges the closing spring. This recharging operation takes several seconds.
It is previously known to use helical springs in spring operated actuators or spring drive units. When the springs are charged or loaded, they are in a compressed state that is not inherently stable. They tend to buckle or roll if they are compressed into the unstable range, and consequently, they require some kind of guiding to prevent buckling. Therefore, in prior art, the springs are enclosed in guiding tubes or cylinders, with a lower end guide plate and the upper end mounted against a frame plate that may be part of the housing for the other parts of the operating device.
It is previously known from JP-A-9259710 to refrain from the use of a guiding tube and allegedly prevent buckling of the spring by instead using other types of guide means, for example one or several guide rods attached to the frame plate and extending down inside the spring to and through a hole in a spring disc at the lower end of the spring. In this way the spring end will certainly still be guided but that does not necessarily mean that the spring does not buckle. This also has the disadvantage of adding extra elements to the structure.
In the currently most common designs of the charging and discharging mechanism of a spring drive unit, the compression and decompression of the opening and closing springs are obtained by a rotary motion of a shaft to which respective spring pullrods are connected. This rotary motion leads to a deflection of the pullrod, and with the traditional designs the deflection, or pullrod angle, of the closing spring is usually larger than for the opening spring. Consequently, the risk for buckling is higher for the closing spring and the demands on the guiding tube are higher.